The present invention relates to pigmented coating compositions comprising starch dispersions as binders, to a novel process for their preparation and to the dispersions obtainable by that process.
Paper is composed of individual fibers and has thus a rough surface and pores whose size corresponds approximately to the fiber dimensions. Since the roughness has a negative effect on printing, most paper and board is coated to impart a smooth surface for printing. Coating of paper is a process by which a mixture of water, white pigments, binder and optionally various additives are applied to one or both sides of a paper sheet. Those pigmented paper coating compositions are also known as coating colors. The resulting covering and smoothening of the surface of the paper improves its glaze, gloss, brightness and above all, its printability in all conventional printing processes.
The properties of the coating colors must be adapted to the coating process and the intended use of the coated paper. The solids content greatly influences the runnability of the coating colors in the coating machine. It basically influences the viscosity of the color and its flow behavior at varying shear stress in the coating machine. The various components of the coating color determine the solids content attainable at a given viscosity. The highest possible solids content is aimed for to save drying energy. For most coating processes the water retention capacity of the coating color is also important; i.e. the ability of the coating color to retain water in spite of the sucking action of the coating base paper. If the water retention capacity of a coating color is too low, it may be too strongly dewatered between application and leveling.
Apart form water. the main components of a coating color are pigments and binders. The function of the binder is to bind the pigment particles to each other and to fix the coat to the base paper. Binders have a large influence on the coating and processing properties of the coating color. Binders are derived from natural raw materials or are prepared fully synthetically. In general, natural binders are used as additives for synthetic polymer binders. Solutions of starch derivatives are a common binder of natural origin. Moreover, the sodium salt of carboxymethyl cellulose (CMC) is the principal cellulose ether used as a naturally derived semi-synthetic co-binder in paper coating.
Whereas in the initial days of paper coating natural binders were used exclusively, in the 1940s the demand for synthetic binders in the form of polymer dispersions (latices) started increasing. These products made it possible for the first time to attain a high solids content at low viscosities, a prerequisite for modern high-speed coating machines. Styrene-butadiene dispersions are the most widely used latices worldwide. Acrylate dispersions are of comparable importance in Europe. These latices are usually mixed with co-binders, such as starch or CMC. A major disadvantage of synthetic and semisynthetic binders is their higher price in comparison to natural binders such as starch.
It was found by the present inventors that starch dispersions can be used as the binder in a pigmented coating composition. The coating composition of the present invention comprises a pigment and a starch dispersion of discrete crosslinked starch particles in an aqueous liquid.
Using the starch dispersions as a binder. storage stable coating compositions can be prepared at relatively high solids contents without significantly high viscosities. The pigmented coating compositions are preferably used as coating colors (pigmented paper coating) to coat paper and cardboard. But they can also be applied to other substrates such as for example plastic films. The pigmented coatings are preferably used to improve paper properties as a precoat or as a topcoat.
Preferably, the starch dispersions are used in the pigmented paper coating compositions as co-binders in combinations with synthetic binders, i.e. polymer latices such as styrene-butadiene dispersions and acrylate dispersions. However, due to the low viscosity of the coating compositions comprising the starch dispersions they may also be used as the sole binder. When the coating composition comprises the starch dispersion as the only binder it is often desirable to use the starch in higher amounts than a synthetic latex in order to obtain the same binding strength.
The pigmented coating compositions comprise the starches preferably in levels from 1 to 300 parts by weight of dry starch based on 100 parts by weight of pigments depending on the specific starch dispersion employed, the desired properties and whether the starch is used as co-binder or the sole binder. If the starch functions as a co-binder it is preferably used in amounts of from 1 to 10 parts by weight in combination with a binder, e.g. a latex, which is used in amounts of from 1 to 20 parts by weight; if the starch is the sole binder it is preferably used in amounts of from 8 to 300 parts by weight, all parts based on 100 parts of pigment.
Preferred pigments are clay, calcium carbonate, calcium sulfoaluminate (satin white), talc, titanium dioxide, barium sulfate, silica or plastic pigments based on polymer particles with a high glass transition temperature like polystyrene.
The pigmented coating composition may contain additional ingredients usually comprised in coating colors. Examples for such additives are foam control agents, optical brightening agents, rheology additives, wetting agents, crosslinking agents, surfactants, lubricants, dispersing agents and dyes.
The advantages of using the starch dispersions according to the present invention as binders in pigmented coating compositions are that higher amounts of starch can be used compared to conventional solutions of starch derivatives and that high solids formulations can be prepared without excessive viscosities and without viscosity instabilities as observed when conventional starch solutions are used in high amounts. The solids content of the pigmented coating compositions preferably ranges from 50 to 74% by weight, more preferably from 55 to 74% by weight and most preferably from 60 to 74% by weight. The pigmented coating compositions simultaneously show superior water retention capacity avoiding runnability deficiencies during coating. Properties of the paper coated with the pigmented coating compositions such as e.g. paper gloss, ink gloss, dry pick resistance, ink piling, ink set off, mottling and brightness are at least as good as with standard coatings if not better. The use of starch as co-binder or sole binder also means a cost advantage versus the use of synthetic binders and co-binders.
Starch can be formed into a dispersion by processing the starch using shear forces and simultaneously crosslinking. A mixture of starch and an aqueous liquid, preferably selected from water and a combination of water and alcohol is processed using shear forces, preferably in an extruder, in the presence of a crosslinker. A hydroxylic liquid, preferably selected from water, alcohol and a mixture of water and alcohol is then added to the mixture and the mixture is dispersed to obtain a dispersion of starch particles.
The starch is preferably native starch, but previously modified starch derivatives may be used as well. Preferred sources of native starch are corn, wheat, rice, potato, tapioca, and barley. The starch can be waxy starch. Starch derivatives which can be used are e.g. cationic and anionic starches, carboxylated starches, carboxy methylated starches, sulfated starches, phosphated starches, starch ethers like hydroxy alkylated starches, e.g. hydroxy ethylated and hydroxy propylated starches, oxidized starches containing carboxy or dialdehyde groups or hydrophobized starches like acetate esters, succinate ester, half-esters or phosphate esters and the like. In the process of preparing the starch dispersion starch granules or pregelatinized starch can be used as preferred starting material. Mixtures of starch with other biopolymers containing at least 50% by weight of starch can also be used as starting material. Suitable biopolymers are exemplified by other polysaccharides such as cellulose and gums. as well as proteins (e.g. gelatin, whey protein).
The starting material is mixed with an aqueous liquid selected from water or a combination from water and an alcohol such as ethanol. A mixture of starch and aqueous liquid also encompasses pregelatinized starch as the starting material which might be fed to the extruder without additional liquid. The mixture preferably has a solids content of at least 50% by weight, especially at least 60% by weight at the time when processing starts.
The processing using shear forces is preferably performed at elevated temperatures above room temperature and below the degradation point of the starch and additional biopolymer, if used. The temperature ranges from preferably 30xc2x0 C., more preferably 40xc2x0 C., and most preferably 60xc2x0 C., to preferably 200xc2x0 C. more preferably 140xc2x0 C. The shear can be effected by applying at least 100 J of specific mechanical energy (SME) per g of starch. Depending on the processing apparatus used the necessary energy may be higher; also when non-pregelatinized material is used. the necessary SME may be higher. preferably at least 250 J/g, more preferably at least 500 J/g. High pressure (e.g. between 5 and 150 bar) may be applied to facilitate processing at high concentrations.
A plasticizer may be present in addition to the water or water/alcohol mixture, such as a polyol (e.g. ethylene glycol, propylene glycol, polyglycols, glycerol, sucrose, maltose, maltodextrines, and sugar alcohols such as sorbitol), urea, sodium lactate, amino acids, or citric acid esters at a level of from 5 to 40% by weight based on the dry weight of the starch or mixture of starch and other biopolymer, if used. However, water can already act as a plasticizer. The total amount of plasticizers (i.e. water and additional plasticizer) preferably ranges from 5 and 50% by weight based on the dry weight of the starch or mixture of starch and other biopolymer, if used. A lubricant, such as lecithin, other phospholipids or monoglycerids, may also be present, preferably at a level of 0.5 to 2.5% by weight based on the dry weight of the starch or mixture of starch and other biopolymer, if used. An acid modifier, preferably a solid or semi-solid organic acid, such as maleic acid, maleic anhydride, citric acid, oxalic acid, lactic acid, gluconic acid, or a carbohydrate-degrading enzyme, such as amylase, may be present at a level of 0.01 to 5% by weight based on the weight of the starch or mixture of starch and other biopolymer; the acid modifier or enzyme assists in slight depolymerization which is assumed to be advantageous in the process of producing starch dispersions comprising particles of a specific size.
An essential step in the process of forming the starch dispersion is the crosslinking. Usually heat and shear treatment of native starches without simultaneously crosslinking leads to starch pastes which are much more viscous at solids contents of higher than 5% and room temperature. For reaching solids contents in the range of 20% at manageable viscosities at room temperature highly modified starches like acid modified starches. oxidized starches and derivatives like hydroxyethyl esters or acetate esters are needed and for example subjected to jet cooking. These high solids slurries are however not storage stable at room temperature and need to be stored at high temperatures in the range of 70 to 90xc2x0 C.
Suitable crosslinkers for preparing the starch dispersions are dialdehydes and polyaldehydes, which reversibly form hemiacetals, acid anhydrides and mixed anhydrides (e.g. succinic and acetic anhydride) and the like. Suitable dialdehydes and polyaldehydes are glutaraldehyde, glyoxal, periodate-oxidized carbohydrates, and the like. Other crosslinkers are for example epichlorohydrin and other epoxides, triphosphates, divinyl sulphone. The crosslinking agent can be a reversible or non-reversible crosslinker. The level of crosslinker varies depending on the strength of the crosslinks it forms with starch. Such crosslinkers may be used alone or as a mixture. The crosslinking reaction may be acid- or base-catalyzed. The level of crosslinking agent preferably ranges from 0.01 to 7% by weight, based on the dry weight of the starch or mixture of starch and other biopolymer, if used. The crosslinking agent is either already present at the start of the shearing process or added during the shearing process. Addition of the crosslinking agent during processing is preferred when the starting material is granular starch.
After crosslinking. dispersion of the mixture in a suitable liquid follows. usually water and/or another hydroxylic liquid such as an alcohol, preferably to a concentration of from 20 to 50% by weight, more preferably from 25 to 50% by weight, and most preferably from 30 to 50% by weight. The final dispersing step is preferably conducted at ambient temperature.
The resulting starch dispersions are low in viscosity, viscosity-stable at ambient temperature and colloidally stable. It is preferred that the viscosities of starch dispersions having a solids content of at least 25% by weight is less than 4,000 mPaxc2x7s, more preferably less than 3,000 mPaxc2x7s and most preferably less than 2,500 mPaxc2x7s, measured at 23xc2x0 C. using a Brookfield RVT viscosimeter at a spindle speed of 20 rpm. The starch dispersions are preferably viscosity-stable for at least two months.
Stable starch dispersions can even be prepared from native starches by processing the starch using shear forces and simultaneously crosslinking as described above. Of course, modified starches may also be used, if desired.
If desired, the dispersed starch may be further crosslinked, using the same or other crosslinking agents as describe above.
A cryogenic grinding step may be performed prior to the dispersion step. However, the starch dispersion for use in pigmented coating compositions can also be prepared by a process wherein the liquid to disperse the starch particles is injected directly into the extruder without a cryogenic grinding step. This novel method of preparing a dispersion of starch particles in an aqueous liquid comprises:
(a) obtaining a mixture of starch and an aqueous liquid;
(b) processing the mixture using shear forces in the presence of a crosslinker in an extruder;
(c) adding a hydroxylic liquid to the extruder and dispersing the mixture inside the extruder or outside the extruder or both inside and outside the extruder to obtain the dispersion.
The novel process which is a preferred embodiment of the general process described above is performed in a way that a premix of starch (e.g. starch/water mixture or starch/water/alcohol mixture) which optionally contains a plasticizer is processed in an extruder. The crosslinker is present in the premix or injected into the extruder. The extrusion is performed under the same conditions as described above for the general process, i.e. a minimum energy input of 100 J/g is needed during the processing. The hydroxylic liquid, preferably water, is injected into the starch mixture in one of the last sections of the extruder at a level to reach the final solids content. Reversed screw elements and kneading elements may be available in the last sections of the extruder in order to homogenize the dispersion. It is also possible to obtain the final dispersion outside the extruder. Preferably, the outside dispersion follows a homogenization step inside the extruder. The preferred methods to obtain the final dispersion are:
the extrudate is stirred in a dispersing vessel (batch type process); or
the extrudate is forced through a tube containing some static mixing elements which homogenize the dispersion (continuous process); or
the extrudate is fed to a continuous pumping device which introduces shear on the heterogeneous flow and thus causing the extrudate to homogenize.
A sieve may be placed just before the die to reduce the size of the starch lumps before they enter the outside dispersing unit.
It is also possible to use already crosslinked starch in the dispersion step. The present invention is thus also directed to a method of preparing a dispersion of starch particles in an aqueous liquid the method comprising extruding a mixture of a crosslinked cationic starch and an aqueous liquid in the presence of a hydroxylic liquid to form the dispersion.
The novel starch dispersions made by the novel process are characterized by a broad particle size distribution. According to laser light scattering data the particle size typically ranges from 200 nm to 100 xcexcm. It is theorized that the broad particle size distribution might be one reason for the exceptionally low viscosity of the starch dispersions.
The present invention will now be illustrated by the following examples. All parts and percentages are based on weight unless otherwise indicated.